Micro light emitting diode array and manufacturing method thereof

ABSTRACT

An embodiment of the present invention provides a micro light emitting diode (LED) array and its manufacturing method. The micro-LED includes a substrate, an epitaxial layer formed on the substrate, and a conversion film formed on the epitaxial layer. Pixels can be defined through lithography, and the pixel size can be very small. This method is characterized in that a mass transfer is not required.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/736,286 filed on Jan. 7, 2020, now allowed, which claimspriority to Taiwan Patent application Ser. No. 108127903, filed on Aug.6, 2019, entitled “Micro Light Emitting Diode Array And ManufacturingMethod Thereof,” which applications are both expressly incorporated byreference herein, in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a micro light-emitting diode array andits manufacturing method.

2. Description of Related Art

Micro light-emitting diode (microLED), also known as “mLED” or “μLED,”is an emerging flat-panel display technology. A micro light-emittingdiode display is composed of an array of micro light-emitting diodesforming individual pixels. Compared to widely used liquid crystaldisplay (LCD), micro light-emitting diode displays provide highercontrast, faster response time, and better energy efficiency.

Organic light-emitting diodes (OLEDs) and micro light-emitting diodescan greatly reduce energy consumption compared to conventional LCDsystems. Unlike OLEDs, microlight-emitting diodes are based onconventional gallium nitride (GaN) light-emitting diode technology,which provides much higher total brightness, up to 30 times, and higherefficiency (lux/W) than OLEDs.

Generally, the dimension of a LED die is between 200 and 300 micrometers(μm), the dimension of a mini light-emitting diode die is about between50 and 100 micrometers, and the dimension of a micro-light emittingdiode die is about 15 microns.

In the manufacturing process of a micro light-emitting diode display, anepitaxial layer having a thickness of about 4-5 μm must be lifted off bya physical or chemical manner and then transferred onto a circuitsubstrate. At present, the most significant challenge of manufacturingμLED is finding ways to place a huge amount of micron-level epitaxiallayers on a target substrate or circuit through a high-precision device,and this is known as “mass transfer.”

Taking a 4K television as an example, the number of epitaxial dies thatneed to be transferred is as high as 24 million. Even if it can betransferred 10,000 dies per time, it needs to be repeated 2,400 times.The yield and efficiency of massive transfers are highly technicallydifficult, so the field is actively researching breakthroughs.

SUMMARY OF THE INVENTION

An object of this invention is to provide a micro light-emitting diodearray and its manufacturing method.

According to an embodiment of this invention, a micro light emittingdiode array is provided with a substrate, a plurality of epitaxiallayers, a plurality of first conversion films, and a plurality of secondconversion films. The epitaxial layers are disposed on the substrate foremitting a first light having a first color. The first conversion filmsare respectively formed on a plurality of first upper surfaces of theplurality of epitaxial layers. The second conversion films arerespectively formed on a plurality of second upper surfaces of theplurality of epitaxial layers. Wherein, each of the first conversionfilm and each of the second conversion film comprise one or morelight-emitting materials and a host. Each light-emitting materialabsorbs the first light and then re-emitting another light with a colordifferent from the first color. The host eliminates grain boundaries andlight scattering of the one or more light-emitting materials after thefirst conversion films or the second conversion films are formed.

According to another embodiment of this invention, a method ofmanufacturing a micro light emitting diode array comprises the steps of:providing a substrate; forming a plurality of epitaxial layers on asurface of the substrate, each of the epitaxial layers emitting a lightof a first color; forming a plurality of first conversion films on aplurality of first upper surfaces of the epitaxial layers; forming aplurality of second conversion films on the plurality of second uppersurfaces of the epitaxial layers; wherein the steps for forming theplurality of first conversion films and the plurality of secondconversion films comprises: dissolving one or more organic dyes and ahost in a solvent to prepare a light-emitting solution; forming thelight-emitting solution on the first upper surfaces or the second uppersurfaces; removing the solvent from the light-emitting solution to formthe first conversion films or the second conversion films; wherein eachorganic dye absorbs the light of the first color and then re-emits alight of a second color or a light of a third color, and the hosteliminates grain boundaries and light scattering of the one or moreorganic dyes form the first conversion films or the second conversionfilms are formed.

The present invention overcomes the problem of massive transfer of microLED arrays in conventional manufacturing processes. In the conventionalmanufacturing process, the dies may be damaged during the transferprocess, and the alignment must be accurate during transferring to thetarget positions. If the transfer yield is not close to 100%, dark spotsor defects will appear on the display, which will render the entiredisplay panel fail.

The conversion films of the present invention are continuous filmswithout grain boundaries. In contrast, conventional quantum dotfluorescent films can clearly observe uneven coating and grainboundaries, which cause the problem of internal absorption loss andlight dispersion after the fluorescent materials are excited by the(blue) light, leading to overall low illumination efficiency. The microlight emitting diode array and the method for manufacturing the sameaccording to the present invention eliminate the mass transfer. Themethod greatly improves the yield and saves a lot of time. In addition,the conversion films of the present invention can be patterned withconventional lithography to define pixels with a size equal to or lessthan 10 μm. Further, the μEDs produced by the present invention are moreadvantageous than OLEDs in brightness, driving power, and PPI (pixelsper inch).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are schematic views showing a method of fabricating amicro light emitting diode array according to an embodiment of thepresent invention.

FIGS. 2A and 2B are spectrums of excitation light of an epitaxial layerwith a conversion film according to an embodiment of the presentinvention.

FIGS. 3A and 3B are spectrums of excitation light of an epitaxial layerwith a conversion film according to another embodiment of the presentinvention.

FIG. 4 is a schematic diagram showing a micro light emitting diode arrayaccording to another embodiment of the present invention.

FIG. 5 is a flow chart showing a method of fabricating a micro lightemitting diode array according to another embodiment of the presentinvention.

FIG. 6A is a scanning electron micrograph showing a cross-sectional viewof a conversion film in accordance with an embodiment of the presentinvention.

FIG. 6B is a scanning electron micrograph showing a top view of aconversion film in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to those specific embodiments ofthe invention. Examples of these embodiments are illustrated inaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to these embodiments. On thecontrary, it is intended to cover alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well-known process operations and components are notdescribed in detail in order not to unnecessarily obscure the presentinvention. While drawings are illustrated in detail, it is appreciatedthat the quantity of the disclosed components may be greater or lessthan that disclosed, except where expressly restricting the amount ofthe components. Wherever possible, the same or similar reference numbersare used in drawings and the description to refer to the same or likeparts.

FIGS. 1A to 1F are schematic views showing a method of fabricating amicro light emitting diode array according to an embodiment of thepresent invention.

Referring to FIG. 1A, a substrate 10 is provided. The substrate 10 mayinclude, but is not limited to, a sapphire substrate, a glass substrate,a silicon substrate, a silicon carbide substrate, a plastic substrate,or other semiconductor substrates. The substrate 10 is cleaned usingnormal procedures well known in the art.

Referring to FIG. 1B, a plurality of epitaxial layers 11 are formed onthe upper surface of the substrate 10 by employing an epitaxial process,e.g., a metal organic chemical vapor deposition (MOCVD) method. By usinga mask (not shown), these epitaxial layers 11 can be formed on thesubstrate 1 at positions where the pixels are formed. The epitaxiallayers 11 can emit light of a first color.

Referring to FIG. 1C, a first mask 12 defining a plurality of openings12 a is formed or disposed on the epitaxial layer 11 to selectivelyexpose the first upper surfaces 11 a of the epitaxial layers 11. Thefirst mask 8 may be a patterned photoresist layer, or may be composed ofother materials such as silicon dioxide or the like. Taking a patternedphotoresist layer as an example, it can be formed using a procedureknown in the art such as photolithography or electron-beam lithography.For example, a photoresist layer is first coated on the epitaxial layers11, and a pattern is transferred to the photoresist layer by performingan exposure with a suitable light source, thereby defining the openings12 a.

In one embodiment, a photoresist S1813 is coated on the epitaxial layer11, followed by soft bake at 115° C. for 3 minutes. Next, thephotoresist is exposed for 18 seconds. Next, the substrate 10 isimmersed in the developer MF-319 for 12 seconds, and then immersed indeionized water for 3 to 5 seconds. Next, the substrate 10 is hard bakedat 125° C. for 1 minute after it is dried. Next, the openings 12 a areformed by reactive-ion etching with the RF power setting to 100 W anddry etching the photoresist using O² gas.

Referring to FIG. 1D, a first conversion film 13 is formed on each firstupper surface 11 a of the epitaxial layers 11. If the first mask 12 is aphotoresist, a protective layer 14 may be formed on the first conversionfilm 13 after the first conversion film 13 is formed. The protectivelayer 14 may be silicon oxide and may be deposited using an electron gun(E-gun) evaporation system. Next, the first mask 12 is removed orstripped using reactive ion etching.

Referring to FIG. 1E, a second mask 15 defining a plurality of openings15 a is formed or disposed on the epitaxial layer 11 to selectivelyexpose the second upper surfaces 11 b of the epitaxial layers 11. Thesecond mask 15 may be a patterned photoresist layer, or may be composedof other materials such as silicon oxide or the like. Taking thepatterned photoresist layer 15 as an example, it can be formed using atechnique known in the art such as optical lithography or electron beamlithography. For example, a photoresist layer is first coated on theepitaxial layer 11, and a pattern is transferred to the photoresistlayer by performing an exposure with a suitable light source, therebydefining the openings 15 a.

In one embodiment, a photoresist S1813 is coated on the epitaxial layer11, followed by soft bake at 115° C. for 3 minutes. Next, thephotoresist is exposed for 18 seconds. Next, the substrate 10 isimmersed in the developer MF-319 for 12 seconds, and then immersed indeionized water for 3 to 5 seconds. Next, the substrate 10 is hard bakedat 125° C. for 1 minute after it is dried. Next, the openings 15 a areformed by reactive-ion etching with the RF power setting to 100 W anddry etching the photoresist using O² gas.

Referring to FIG. 1F, a second conversion film 16 is formed on eachsecond upper surface 11 b of the epitaxial layers 11. Next, the secondmask 15 is removed or stripped using reactive ion etching.

The conversion films provided by the present invention, such as thefirst conversion film 13 and the second conversion film 16, are films atleast composed of one or more light-emitting materials and a host.Preferably, the conversion films are produced by a solution method. Boththe one or more light-emitting materials and the host are dissolved in asolvent to form a light-emitting solution, which is then formed on theneeded positions, such as the first upper surface 11 a or the secondupper surface 11 b.

After that, a first conversion film 13 is formed on the first uppersurface 11 a by removing (e.g., drying) the solvent from thelight-emitting solution. Or, a second conversion film 16 is formed onthe second upper surface 11 b by removing (e.g., drying) the solventfrom the light-emitting solution. In particular, the host is used tokeeps the optical performance of light-emitting materials in theconversion film as it in the liquid form. In addition, the host caneliminate grain boundaries and light scattering of the light-emittingmaterials after the conversion film is formed. According to embodimentsof this invention, the light-emitting materials are photoluminescentmaterials which absorb a light with a first color and re-radiate a lightwith a second color. Preferably, the light-emitting materials areorganic dyes comprising non-rare earth elements. In one embodiment, thehost keeps the polarity of the organic dyes and hence keeps absorptionand radiation wavelength range as it in the liquid form.

In one embodiment, the host comprises a liquid form of silicon dioxide.In one embodiment, the host comprises a polymer having a goodfilm-forming and cladding properties. In one embodiment, the polymercomprises polyvinylpyrrolidone (PVP), epoxy, polymethylmethacrylate(PMMA), or polydimethylsiloxane (PDMS). In one embodiment, the solventcomprises ethanol, chloroform, dichloromethane, or other solventscapable of dissolving the one or more organic dyes and the polymer. Inone embodiment, the weight ratio of the organic dyes to the polymerranges between 1:200 and 1:20000. In one embodiment, method for formingthe light-emitting solution on a needed position (such as the firstupper surface 11 a and the second upper surface 11 b) may comprise, butis not limited to, spin coating, dip coating, ink jet printing, screenprinting, comma coating, or roll coating. In one embodiment, thelight-emitting solution is formed on a needed position by spin coatingand the coating time is between 10 sec and 3 min. Next, the solvent isremoved from the light-emitting solution so as to form a conversionfilm, e.g., the first conversion film 13 or the second conversion film16. In one embodiment, the solvent can be removed from thelight-emitting solution by natural (air) seasoning or other manners. Thefirst conversion film 13 or the second conversion film 16 is formed oncethe solvent is removed.

In the embodiment shown in FIGS. 1A-1F, the epitaxial layers 11 can emita blue light, and the first conversion films 13 absorb the blue lightemitted from the epitaxial layers 11 and then emit a green light. Inaddition, the second conversion films 16 absorb the blue light emittedfrom the epitaxial layers 11 and then emit a red light. Embodiments ofthe invention are not limited thereto and may have other arrangements.

An embodiment of preparing the first conversion film 13 is exemplifiedbelow.

Firstly, an organic dye, C545T, is dissolved with a proper solvent,e.g., ethanol. In other embodiments, the solvent ethanol can be replacedby another solvent capable of dissolving the organic dye C545T. The fullname of C545T is10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyropyrano(6,7-8-I,j)quinolizin-11-one.

After that, the solution of C545T and solvent is agitated for 30 min sothat C545T is completely dissolved and a light-emitting solution capableof emitting green light is formed. A polymer, such aspolyvinylpyrrolidone (PVP), is then added into the above light-emittingsolution.

After that, a heating plate is preheated to 60° C. and then used to heatthe light-emitting solution. During the heating, the light-emittingsolution is agitated until the polyvinylpyrrolidone is completelydissolved.

The light-emitting solution capable of emitting green light is thenspin-coated on the target positions (e.g., the first upper surfaces 11a) with a speed between 500 rpm and 9000 rpm for 10 sec.

After that, the substrate 10 is placed under atmosphere, so as toevaporate the solvent from the light-emitting solution and thusgradually form a first conversion film 13 capable of emitting greenlight. Finally, a protective layer 14 may be coated on the firstconversion film 13.

An embodiment of preparing the second conversion film 16 is exemplifiedbelow.

Firstly, an organic dye, DCJTB, is dissolved with a proper solvent,e.g., dichloromethane. In other embodiments, the solvent ethanol can bereplaced by another solvent capable of dissolving the organic dye DCJTB.The full name of DCJTB is2-tert-Butyl-4-(dicyanomethylene)-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)vinyl]-4H-pyran.

After that, the solution of DCJTB and solvent is agitated for 30 min sothat DCJTB is completely dissolved and a light-emitting solution capableof emitting red light is formed. A polymer, such as polyvinylpyrrolidone(PVP), is then added into the above light-emitting solution.

After that, a heating plate is preheated to 60° C. and then used to heatthe light-emitting solution. During the heating, the light-emittingsolution is agitated until the polyvinylpyrrolidone is completelydissolved.

The light-emitting solution capable of emitting red light is then spincoated on the target positions (e.g., the second upper surfaces 11 b)with a speed between 500 rpm and 9000 rpm for 10 sec.

After that, the substrate 10 is placed under atmosphere, so as toevaporate the solvent from the light-emitting solution and thusgradually form a second conversion film 16 capable of emitting redlight.

FIG. 2A shows an emission spectrum in which the epitaxial layer emitsblue light and a green light excitation spectrum excited by the bluelight in the above embodiment. FIG. 2B is the excited green lightspectrum in FIG. 2A showing the emission band of the excited light.

FIG. 3A shows an emission spectrum in which the epitaxial layer emitsblue light and a red light excitation spectrum excited by the blue lightin the above embodiment. FIG. 3B is the excited red light spectrum inFIG. 3A showing the emission band of the excited light.

Although the conversion films of the above-mentioned embodiment emits asingle color light within a wavelength interval, in other embodimentstwo or more organic dyes may be used so that the produced conversionfilm can emit two or more color light beams with one or more wavelengthintervals.

A person skilled in the art can make various modifications,substitutions, or alterations to the embodiments shown in FIGS. 1A-1F,and such modifications, substitutions, or alterations are within thescope of the invention. For example, FIG. 4 shows a micro light emittingdiode array in accordance with another embodiment of the presentinvention. In this embodiment, the epitaxial layers 11 emit anultraviolet light, and the plurality of third conversion films 17 areformed on the plurality of third upper surfaces 11 c of the epitaxiallayers 11. After absorbing the ultraviolet light, the first, second, andthird conversion films respectively emit another color of light, such asa green light, a red light, and a blue light. In another embodiment, aplurality of fourth conversion films (not shown) are further formed on aplurality of fourth upper surfaces (not shown) of the epitaxial layers11. In one embodiment, the epitaxial layers 11 and/or at least one kindof conversion film may be integrally formed on the substantially wholesurface of the substrate 10, and then the integrally formed epitaxiallayers 11 and/or the conversion film is patterned by lithography todefine pixels.

FIG. 5 is a flow chart showing a method of fabricating a micro lightemitting diode array according to another embodiment of the presentinvention. Referring to FIG. 5, the method includes: step 51, providinga substrate; step 52, forming a plurality of epitaxial layers on asurface of the substrate, each of the epitaxial layers emitting a lightof a first color; step 53, forming a plurality of first conversion filmson a plurality of first upper surfaces of the epitaxial layers; and step54, forming a plurality of second conversion films on a plurality ofsecond upper surfaces of the epitaxial layers. A method for forming theplurality of first conversion films or the plurality of secondconversion films may comprise the steps of: dissolving one or moreorganic dyes and a host in a solvent to prepare a light-emittingsolution; forming the light-emitting solution on the first uppersurfaces or the second upper surfaces; removing the solvent from thelight-emitting solution to form the first conversion films or the secondconversion films. Wherein, each organic dye absorbs light of the firstcolor and then re-emits a light of another color. In addition, the hosteliminates the grain boundaries and light scattering of the one or moreorganic dyes after the first conversion films or the second conversionfilms are formed. The order of the described steps in this embodimentcan be changed.

FIGS. 6A and 6B are scanning electron microscope images showingcross-sectional view and top view of a conversion film (with a thickness5.76 μm) produced according to an embodiment of the present invention.As shown in FIGS. 6A and 6B, the conversion film produced by the presentinvention does not have grain boundaries. By contrast, a film formed byconventional fluorescent materials includes grain boundaries. Forapplications of light emitting diode display, because the formed filmincludes grain boundaries, the size of pixel cannot be too small. Forexample, if the average grain size of the fluorescent particles is 10μm, in order to make the brightness of the individual pixels uniform, itis necessary to increase the number of fluorescent particles per pixel,for example, 100 fluorescent particles per pixel. This will result in alarge size of unit pixel, usually greater than 100 μm.

In contrast, each conversion film produced by the present invention is acontinuous film because it does not have grain boundaries. Therefore,the size per unit pixel (i.e., the size of the epitaxial layer) is notlimited to the average brightness and can be arbitrarily defined. Insome embodiments, the size per unit pixel is equal to or less than 15μm. In some embodiments, the size per unit pixel is equal to or lessthan 10 μm. In some embodiments, the size per unit pixel ranges from 1μm to 10 μm. In some embodiments, the size per unit pixel is equal to orless than 5 μm.

Further, because each conversion film of the present invention is acontinuous film without grain boundaries, it is possible to definepixels by patterning the conversion film with a conventionallithography. Alternatively, the conversion film of the present inventionmay be directly formed on the first upper surface, the second uppersurface, and/or the third upper surface of the epitaxial layers.Accordingly, the method provided by the present invention does notrequire a huge amount of transfer and thus can significantly savemanufacturing costs.

The intent accompanying this disclosure is to have each/all embodimentsconstrued in conjunction with the knowledge of one skilled in the art tocover all modifications, variations, combinations, permutations,omissions, substitutions, alternatives, and equivalents of theembodiments, to the extent not mutually exclusive, as may fall withinthe spirit and scope of the invention. Corresponding or relatedstructure and methods disclosed or referenced herein, and/or in any andall co-pending, abandoned or patented application(s) by any of the namedinventor(s) or assignee(s) of this application and invention, areincorporated herein by reference in their entireties, wherein suchincorporation includes corresponding or related structure (andmodifications thereof) which may be, in whole or in part, (i) operableand/or constructed with, (ii) modified by one skilled in the art to beoperable and/or constructed with, and/or (iii) implemented/made/usedwith or in combination with, any part(s) of the present inventionaccording to this disclosure, that of the application and referencescited therein, and the knowledge and judgment of one skilled in the art.

Conditional language, such as, among others, “can,” “could,” “might,” or“may,” unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey thatembodiments include, and in other interpretations do not include,certain features, elements and/or steps. Thus, such conditional languageis not generally intended to imply that features, elements and/or stepsare in any way required for one or more embodiments, or interpretationsthereof, or that one or more embodiments necessarily include logic fordeciding, with or without user input or prompting, whether thesefeatures, elements and/or steps are included or are to be performed inany particular embodiment.

All of the contents of the preceding documents are incorporated hereinby reference in their entireties. Although the disclosure herein refersto certain illustrated embodiments, it is to be understood that theseembodiments have been presented by way of example rather thanlimitation. For example, any of the particulars or features set out orreferenced herein, or other features, including method steps andtechniques, may be used with any other structure(s) and processdescribed or referenced herein, in whole or in part, in any combinationor permutation as a non-equivalent, separate, non-interchangeable aspectof this invention. Corresponding or related structure and methodsspecifically contemplated and disclosed herein as part of thisinvention, to the extent not mutually inconsistent as will be apparentfrom the context, this specification, and the knowledge of one skilledin the art, including, modifications thereto, which may be, in whole orin part, (i) operable and/or constructed with, (ii) modified by oneskilled in the art to be operable and/or constructed with, and/or (iii)implemented/made/used with or in combination with, any parts of thepresent invention according to this disclosure, include: (I) any one ormore parts of the above disclosed or referenced structure and methodsand/or (II) subject matter of any one or more of the inventive conceptsset forth herein and parts thereof, in any permutation and/orcombination, include the subject matter of any one or more of thementioned features and aspects, in any permutation and/or combination.

Although specific embodiments have been illustrated and described, itwill be appreciated by those skilled in the art that variousmodifications may be made without departing from the scope of thepresent invention, which is intended to be limited solely by theappended claims.

What is claimed is:
 1. A micro light emitting diode array, comprising: asubstrate; a plurality of epitaxial layers in direct contact with asurface of the substrate for emitting a first light having a firstcolor; a plurality of first conversion films formed on a plurality ofexposed first upper surfaces of the plurality of epitaxial layers, eachof the plurality of first conversion films being in direct contact withrespective one of the plurality of exposed first upper surfaces; and aplurality of second conversion films formed on a plurality of exposedsecond upper surfaces of the plurality of epitaxial layers, each of theplurality of second conversion films being in direct contact withrespective one of the plurality of exposed second upper surfaces;wherein each of the first conversion film and each of the secondconversion film comprise: one or more light-emitting materials, eachlight-emitting material absorbing the first light and then re-emittinganother light with a color different from the first color; a host thateliminates grain boundaries and light scattering of the one or morelight-emitting materials after the first conversion films or the secondconversion films are formed.
 2. The micro light emitting diode array asrecited in claim 1, wherein each of the epitaxial layers defines a pixelhaving a size equal to or less than 15 μm.
 3. The micro light emittingdiode array as recited in claim 1, further comprising a plurality ofthird conversion films respectively formed on a plurality of third uppersurfaces of the epitaxial layers.
 4. The micro light emitting diodearray as recited in claim 1, wherein the one or more light-emittingmaterials comprise organic dyes made of non-rare earth elements, and thehost keeps the polarity of the organic dyes and hence keeps absorptionand radiation wavelength range as in the liquid form.
 5. The micro lightemitting diode array as recited in claim 4, wherein the organic dyescomprise C545T or DCJTB.
 6. The micro light emitting diode array asrecited in claim 1, wherein the host comprises a polymer.
 7. Thelight-emitting thin film as recited in claim 6, wherein the polymercomprises polyvinylpyrrolidone (PVP), epoxy, polymethylmethacrylate(PMMA), or polydimethylsiloxane (PDMS).